Photobiological Fuel Production and CO2 Fixation

Motivation

Industrial nations in general and the United States of America in particular are facing an unprecedented combination of economic and environmental challenges. First, they face the formidable challenge to meet expanding energy needs without adding intolerable amounts of greenhouse gases to the atmosphere and further impacting climate and the environment. The reserves of cheap natural resources the world has been relying on for decades are now estimated in tens of years. Evidence of global warming already gathered around the globe and, most likely, due to industrial activities will put an additional stress on the fragile balance we have been enjoying. In order to face these formidable challenges and to create technological and economic opportunities, the United States should reduce its dependency on foreign fossil fuels and rely more on a combination of (i) sustainable energy conversion and transportation systems, (ii) oil-free energy sources, and (iii) new technologies for capturing and converting carbon dioxide.

Photosynthesis

Photosynthesis begins with the absorption of photons by the photosynthetic apparatus which consists of three major components (i) the reaction center, (ii) the core antenna, and (iii) the peripheral antenna. Photochemical charge separation and electron transport take place in the reaction center. The core antenna contains the photosynthetic pigments chlorophylls or bacteriochlorophylls. It is surrounded by the peripheral antenna which is an assembly of chlorophylls, bacteriochlorophylls, and other accessory pigments such as carotenoids and phycobiliproteins. The peripheral antenna is particularly important in channeling additional photon energy to the reaction center at small light intensities. In microalgae and cyanobacteria, the photosynthetic apparatus is located on the photosynthetic membrane called thylakoid. Different pigment molecules absorb over different spectral bands of the visible and near infrared parts of the spectrum enabling more efficient utilization of solar energy.

Fig. 1 shows the in vivo specific absorption coefficient Ea (in m2/mg) of primary pigments chlorophylls a, b, and c as well as accessory pigments such as photosynthetic carotenoids (PSC), and photoprotective carotenoids (PPC) measured over the spectral region from 400 to 750 nm. It indicates that Chlorophyll a (Chl a) absorbs around 435 and 676 nm while Chlorophyll b (Chl b) absorbs around 475 and 650 nm. Since they do not absorb green light (λ = 520-570 nm) significantly, these microalgae appear green to the human eye. On the other hand, carotenoids are accessory pigments found in all photosynthetic microorganisms. They absorb mainly in the blue part of the spectrum (400 < λ < 550 nm).
Carotenoids serve two major functions (i) shielding the photosynthetic apparatus from photo-oxidation under large light intensities and (ii) increasing the solar light utilization efficiency by expanding the absorption spectrum of the microorganism.

Cyanobacteria produce hydrogen and oxygen by (i) consuming the CO2 gas as their carbon source and (ii) absorbing solar light as their energy source.

Objectives

The objective of our effort is to perform a comprehensive study to simultaneously mitigate carbon dioxide and produce biofuels. It offers a cheap, efficient, scalable, autonomous, and reliable system for producing hydrogen from microbial consumption of carbon dioxide and absorption of solar light.